PROCEDIMIENTO PARA INCLUSIÓN DE ÍTEMS NO PACTADOS

ATOMIC LAYER DEPOSITION OF TUNGSTEN-RICH TUNGSTEN CARBIDE FILM USING WCl6 AND AlH2(tBuNCH2CH2NMe2) AS PRECURSORS

Reprinted (adapted) with permission from Blakeney, K. J.; Ward, C. L.; Winter, C. H. ECS

Trans. 2018, 86, in press. Copyright 2018 The Electrochemical Society. 5.1 Introduction

Tungsten-based thin films are essential components of microelectronic devices due to their high melting points, high densities, and low resistivities. Tungsten carbides have been heavily investigated as diffusion barriers,51,179,191,196–198 _{and also as work function metals and}

adhesion layers.177,199 Tungsten carbides are also important catalytic materials.46–48 These reports most often use physical or chemical vapor deposition (PVD or CVD) to deposit the thin films. Atomic Layer Deposition (ALD) is an alternative technique for depositing highly conformal and uniform thin films.6,7,16,165 ALD is a variation of CVD where the growth surface is exposed sequentially to two or more precursors which are separated in time by inert gas purge periods. Film growth is based on self-limiting surface chemical reactions which deposit approximately one monolayer of material with each cycle. ALD is used in the microelectronics industry to fabricate transistors with atomic level precision.200 Other fields such as catalysis have recently began to employ ALD to fabricate precise structures.4,201

Industrial tungsten-based film growth is dominated by WF6 as the tungsten precursor.

Tungsten metal deposited by ALD from WF6 is increasingly used in high aspect ratio contact

holes and gate trenches.39,158 Unfortunately, WF6 readily etches Si surfaces through formation of

strong Si-F bonds and conversion to volatile fluorosilanes. To combat this facile reaction, an etch stop layer, such as TiN, must be used to prevent undesirable pitting of the Si substrate. It is desirable to eliminate extra processing steps and unnecessary materials from integrated circuits

due to the miniaturization demands of Moore’s Law.2 _{Additionally, WF}

6 hydrolyzes to form

toxic and corrosive HF, which presents serious safety concerns for the handling of this precursor.36,187–190 Fluorine-free tungsten processes would therefore have immediate applications in semiconductor manufacturing.183,192,199

As part of a project to develop new reducing precursors and ALD processes for electropositive metal and metal-containing films, Chapter 2 reports the aluminum dihydride complex 1 and its use in ALD of aluminum metal films.170 The structure of 1 is shown in Figure 31. In Chapter 3, where 1 was tested with TiCl4 as the metal precursor, titanium metal films were

not deposited. Instead, titanium carbonitride films were deposited, which were highly air stable and conductive when grown above 180 °C.70 Thermal ALD of pure metallic films of early transition metals is extremely challenging. Tungsten has a much more positive electrochemical potential than titanium (W3+ ↔ W0 E° = 0.100 V, Ti2+ ↔ Ti0 E° = -1.628 V), which prompted an exploration of tungsten ALD film growth using 1.

Figure 31. Structure of the co-reactant 1.

Herein, this chapter reports the thermal ALD growth of tungsten-rich tungsten carbide films using WCl6 and 1. The films are deposited with a growth rate of 1.6 Å/cycle within an

ALD window of 275-350 °C. High purity tungsten carbide films are deposited with a tungsten:carbon ratio between 2.8-3.7, which is greater than the tungsten-rich W2C phase. The

observed self-limited growth and low aluminum content (<1 at. %) show that aluminum hydride complexes such as 1 are promising co-reactants for thermal ALD growth of transition metal and metal-containing films. Rational design of improved derivatives of 1 could reduce carbon incorporation and yield high purity metallic films.

5.2 Experimental Section

ALD experiments were conducted on a Picosun R-200 system using nitrogen carrier gas (5N, Airgas) that was passed through an in-line gas purifier (SAES, <100 ppt H2O, O2). The

operating pressure was 5-8 Torr. WCl6 was purchased from Strem Chemicals and was purified

by sublimation. Thermogravimetric analysis (TGA) was performed on a TA Instruments SDT- 2960 TGA with a heating rate of 10 °C/min. Complex 1 was synthesized as described previously.170 _WCl

6 and 1 were heated to 125 and 55 °C, respectively, using Picosolid boosters

(bubbler-style precursor source) and pulsed into the deposition chamber with inert gas valving. Films were deposited on Si(100) substrates which were cleaned by the standard RCA method.202 Some films were deposited on uncleaned Si(100) with native oxide and SiO2 (100 nm thermal

oxide on Si) substrates. Film thickness was measured primarily by cross-sectional scanning electron microscopy (SEM) using a JEOL 7600 FE-SEM. X-ray reflectivity (XRR) and grazing- incidence X-ray diffraction (GIXRD) measurements were performed with a Bruker D8 Advance diffractometer using Cu Kα (1.5418 Å) radiation at 40 kV/40mA tube power. Thin film samples were rigorously aligned using Z-height beam splitting and rocking curve scans (2-3 iterations). Annealing studies were carried out in an Anton-Paar XRK-900 XRD chamber at 5x10-2 Torr. XRR patterns were modeled using open-source GenX (v2.4.10) software. Resistivity was

measured using a four-point probe (Keithley 2400 sourcemeter, Keithley 2182A nanovoltmeter, Jandel four-point probe). Film elemental composition was investigated by X-ray photoelectron spectroscopy (XPS) using a Kratos Axis Ultra XPS system with a monochromatic Al Kα (1486.6 eV) source. Argon ion sputtering (3.8 kV) was used for depth profiling and to remove surface adventitious carbon and oxygen. XPS spectra were analyzed with CasaXPS software.

5.3 Results and Discussion

Precursor Properties. WCl6 is a solid with a vapor pressure of 9.75 Torr at 190 °C.203 For

comparison, HfCl4, which has achieved widespread adoption as an industrial ALD precursor, is a

solid with a vapor pressure of 1 Torr at 190 °C.204 The TGA plot of WCl6 shows a mass loss

event between 150-230 °C which is visible in both the TGA and differential thermogravimetric (DTG, first derivative) curves (Figure 32). There is only 0.78% residual mass, which indicates clean volatilization of WCl6 without decomposition. There is a small amount of mass loss at

~120 °C, which is likely volatile tungsten oxychloride impurity caused by brief air exposure during sample loading.

Figure 32. TGA and DTG (first derivative) curves for WCl6. A small amount of tungsten

oxychloride is present due to brief air exposure during sample loading.

Chapter 2 reports a volatile aluminum dihydride complex coordinated by a bulky tert- butylamido-amine ligand (AlH2(tBuNCH2CH2NMe2), 1).170 Complex 1 has improved thermal

stability compared to other volatile aluminum hydride complexes, which enabled a self-limiting thermal ALD process for aluminum metal films.170 The vapor pressure of 1 is 0.75 Torr at 70 °C, its solid state thermal decomposition temperature is ca. 185 °C, and it has low melting point of 31-32 °C.170 Complex 1 also displays a quantitative, single-step weight loss in its TGA curve.170 Thus, WCl6 and 1 exhibit good ALD precursor characteristics.

ALD Film Growth. Thin film deposition experiments using WCl6 and 1 were carried out

between 150-400 °C. WCl6 and 1 were evaporated at 125 °C and 55 °C, respectively, and were

sequentially pulsed into the deposition chamber separated by 10 s nitrogen purge periods. Films were deposited on 1-2 cm2 coupons of Si(100) cleaned by the standard RCA method. Films deposited at 150 and 200 °C for 250 cycles had sheet resistivities > 106 /square and were highly unstable in air, becoming powdery after several minutes. Poor film growth below 200 °C

was also observed when using TiCl4 as the metal precursor (Chapter 3).70 Higher deposition

temperatures afforded stable, reflective, silvery-grey films which allowed for a study of ALD process characteristics.

Pulse lengths of both precursors were varied at a deposition temperature of 300 °C to determine if the film growth proceeds via self-limited steps. First, the pulse length of 1 was varied while the pulse length of the WCl6 was constant (3 s) (Figure 33a). A 1 s pulse length

yielded a lower growth rate of 1.1 Å/cycle while the growth rate saturated for pulse lengths ≥ 2 s

at 1.5 Å/cycle. The pulse length of WCl6 was then varied while keeping the 1 pulse length

constant at 3 s (Fig. 3b). A 1 s pulse produced a much lower growth rate of 0.6 Å/cycle, while pulse lengths ≥ 2 s yielded identical growth rates of 1.5 Å/cycle. Chapter 4 reports similar

saturation characteristics for WCl6.54 A control experiment was conducted at 300 °C with 300

cycles in which 1 (3 s) was pulsed into the chamber, but no WCl6 pulses were used. No film

growth was detected by SEM. A similar experiment was carried out at 300 °C with 300 cycles of WCl6 (3 s) pulses. Again, no film growth was observed by SEM. The observed saturative

behavior of 1 at 300 °C is remarkable, since the solid state thermal decomposition point of 1 is 185 °C.170 Accordingly, 1 must undergo thermal decomposition during the film growth, but the reaction products must be able to transfer the ligand from 1 to a surface-bound tungsten species to afford carbon in the films. Detailed insights into the mechanism of carbon incorporation will require additional experimentation.

Figure 33. Growth rate versus precursor pulse length for (a) 1 and (b) WCl6 at 300 °C.

Using a saturative pulse sequence of WCl6 (3 s), N2 purge (10 s), 1 (3 s), N2 purge (10 s),

growth rate was measured as a function of temperature between 250-400 °C after 300 cycles (Figure 34). An ALD window, where growth rate was approximately independent of temperature, was observed between 275-350 °C with a growth rate of 1.6 Å/cycle. Growth rate increased at temperatures > 350 °C which could be related to increased thermal decomposition of

Figure 34. Growth rate versus temperature after 300 ALD cycles.

A plot of film thickness versus number of cycles at 300 °C was linear between 35-400 cycles (Figure 35). Film thicknesses measured by both cross-sectional SEM and XRR were in agreement. The slope of the trendline fitted to XRR thickness measurements was 1.607 Å/cycle which is consistent with the growth rates measured after 300 cycles for the saturation and ALD window plots. The y-intercept of the trendline was close to zero, indicating negligible nucleation delay on RCA-cleaned Si substrates.

Figure 35. Film thickness versus number of ALD cycles at 300 °C.

Film Structure and Composition. XRR was used as a secondary thickness measurement

and also to determine film density and surface roughness (Figure 36). The XRR model is described pictorially in Figure 6 which included a thin interfacial SiO2 layer, but the fit was not

improved by adding an oxidized tungsten surface layer, so this was not included. The model and calculated film densities were based on a film composition of W3C. Root mean square surface

roughness values (σ) decreased with increasing film thickness from 9.0 Å (93 Å thick film) to 1.8 Å (647 Å thick film). Film densities (ρ) ranged from 9.1-7.5 g/cm3 for films deposited at 300 °C between 93 and 648 Å thick. These densities are 50-60 % of the ideal bulk densities of WC (15.7 g/cm3) and W2C (17.3 g/cm3).36,199 In general, densities of films grown by thermal ALD

tungsten carbonitride films can be deposited using WF6, B2H6 or BEt3, and NH3 with densities of

15 and 15.4 g/cm3, respectively.189,191 By contrast, a report of thermal ALD of NbCx films from

NbF5 and AlMe3 reported densities roughly 50 % of bulk NbC.185

Figure 36. XRR analysis for films deposited on Si using WCl6 and 1 at 300 °C after 65 (top), 100

(middle), and 400 (bottom) cycles.

Film crystallinity was evaluated using GIXRD. The as-deposited films displayed a broad reflection centered at 2θ = 37.5°, which is a common feature of vapor deposited tungsten carbide

films (Figure 37).51,179,183,191,199,205 _{A film deposited at 300 °C for 300 cycles was annealed under}

vacuum and its structure was evaluated between 600-900 °C. The film structure converts to the non-stoichiometric β-WC1-x phase above 600 °C.183,199,205,206 At 900 °C, the structure changes

significantly and strong reflections are observed at 2θ = 40 and 58°, which match cubic tungsten metal. The major reflection at 2θ = 40° also matches the (111) reflection of W2C. A similar

annealing experiment of a tungsten carbide film deposited at 300 °C using WCl6 and AlMe3 also

produced reflections corresponding to tungsten metal (Data not shown). Trace amounts of oxygen present in the films or in the low vacuum of the annealing chamber (5 x 10-2 Torr) could allow removal of carbon from the films, forming CO2 and tungsten metal. No evidence for

tungsten silicide formation was observed, which is similar to previous annealing experiments of ALD WCN films.191

Figure 37. GIXRD patterns of a film deposited from WCl6 and 1 at 300 °C after 300 cycles.

Reference patterns are included for α-W metal (blue lines, PDF#04-0806) and β-WC1-x (green

lines, PDF#20-1316). Weak reflections at 2θ = 21° and 54° were indexed to WO3 (reference not

Film composition according to XPS after Ar ion sputtering to remove surface contamination showed tungsten and carbon as the major components with low levels of nitrogen (< 4 at.%), oxygen (< 4 at.%), chlorine (< 3 at.%), and aluminum (< 1 at.%) (Figure 38). The tungsten:carbon ratio x for WxC after sputtering was 2.8-3.7, which is higher than the more

common W2C phase, and indicates that this ALD process produces tungsten-rich films. The high

resolution core level scans are shown in Figure 8. The W 4f7/2 ionization was between 31.5-31.4

eV when the C 1s ionization was calibrated to 283.2 eV. For comparison, the previous report of WC ALD in Chapter 4using WCl6 and AlMe3 had W 4f7/2 and C 1s binding energies of 31.6 and

283.2 eV, respectively.54 W metal has a 4f7/2 binding energy of 31.4 eV and, therefore, the

tungsten-rich films deposited herein display a W 4f7/2 binding energy which is intermediate

between tungsten metal and WC films. Additionally, previous reports of transition metal carbide ALD processes have observed two ionizations in the C 1s region at ~283.2 and 284.2 eV that have been attributed to metal carbide and amorphous carbon species, respectively.54,185 _{For this}

tungsten carbide ALD process the C 1s region showed a single ionization (283.2 eV) corresponding to W-C and no evidence for amorphous carbon within the films.

Figure 38. (Left) XPS depth profile of a film deposited from WCl6 and 1 after 250 cycles at 300

°C. (Right) High resolution XPS scans of W 4f, C 1s, N 1s, and Al 2p regions after sputtering.

Although the film composition is tungsten-rich and high purity tungsten carbide, film resistivities were much higher than bulk values. The low film densities as measured by XRR likely explain these higher than expected resistivities. No clear trend with temperature was observed. Resistivities were 1140 Ω·cm at 300 °C, 840 Ω·cm at 325 °C, and 900 Ω·cm at 400 °C for films deposited after 300 cycles on Si. Resistivities were slightly higher on SiO2:

1800 Ω·cm at 300 °C and 1600 Ω·cm at 350 °C. For comparison, tungsten carbide ALD films

grown on SiO2 from WCl6 and AlMe3 had resistivities of 2770 Ω·cm at 300 °C and 1500

Ω·cm at 375 °C.54 _{The tungsten carbide films reported herein have lower resistivities than the}

previously reported tungsten carbide films deposited from WCl6 and AlMe3, which is consistent

with the higher tungsten content, but not as conductive as the tungsten carbide films deposited from WF6 and H2SiEt2, which measured 400-500 Ω·cm.17,24

Mechanistic Speculation. The only source of carbon in this ALD process is the amido-

1 to the WClx growth surface. It is also possible that a decomposition product of 1 transfers the

ligand to the WClx growth surface. Figure 9 shows a possible mechanism for carbon

incorporation into the growing tungsten film. After ligand transfer to the growth surface, β- methyl elimination from the tert-butyl group may be a facile decomposition pathway for the putative tungsten amido-amine surface species. This would form a methyl-terminated tungsten surface and eliminate a neutral amino-imine as a by-product. Chapter 4 on tungsten carbide ALD from WCl6 and AlMe3 described a possible mechanism for carbide formation from a tungsten-

methyl surface based on α-hydride elimination.54

Figure 39. Possible mechanism for carbon incorporation by ligand transfer from 1 to the growth surface and subsequent decomposition and carbide formation.

It is unclear why the ALD process described herein from WCl6 and 1 produces films with

low nitrogen content (< 4 at.%), but the report in Chapter 3 using TiCl4 and 1 produced titanium

surface intermediates, differences in ligand decomposition pathways must exist to account for the clearly different film compositions. The much lower degree of carbon and nitrogen incorporation observed herein (~25 at.% versus ~50 at.% for TiCN) implies that there is less ligand transfer from 1 to the growth surface when using WCl6 as the metal precursor instead of

TiCl4. Future synthetic work will be directed towards designing ligands which can produce

volatile and thermally stable aluminum hydride complexes, but which will not readily transfer to the growth surface and incorporate into the film under ALD conditions, and could therefore produce a pure metal film.

5.3 Conclusions

A thermal ALD process is described using WCl6 and the aluminum dihydride complex

AlH2(tBuNCH2CH2NMe2) (1) as precursors. The process displays an ALD window between

275-350 °C with a growth rate of 1.6 Å/cycle. Film composition according to XPS is high purity tungsten carbide (WxC, x = 2.8-3.7) with low levels of nitrogen, oxygen, chlorine, and aluminum

impurities (< 4, 4, 3, 1 at.%, respectively). The as-deposited films are nanocrystalline and are converted to the non-stoichiometric β-WC1-x phase above 600 °C. Film densities were 50-60% of

bulk tungsten carbide values, which may explain the unexpectedly high resistivities (1140 Ω·cm at 300 °C) for the tungsten-rich films.